chapter 4 flip-chip light-emitting-diodes with textured ... · flip-chip light-emitting-diodes with...
TRANSCRIPT
20
Chapter 4
Flip-Chip Light-Emitting-Diodes with Textured Micro Pillar
Arrays (MPAFC-LEDs)
4-1 Fabrication of MPAFC-LEDs
4-1.1 Process procedure
The GaN LED structure with dominant wavelength at 460 nm used in this
study is grown by metal-organic chemical vapor deposition (MOCVD) on
c-plane sapphire substrates. The LED structure consists of a 2-μm-thick
undoped GaN layer, a 2-μm-thick highly conductive n-type GaN layer, a
0.2-μm-thick InGaN/GaN MQW, a 0.2-μm-thick p-type GaN layer and n
InGaN/GaN short period super-lattice (SPS) tunneling contact layers for
indium-tin-oxide (ITO).
Figure 4-1 shows the fabrication steps of flip-chip GaN LEDs with
micro-pillar-arrays. Top-emitting LEDs with a size of 1000 μm × 1000 μm are
fabricated using standard photolithography and BCl3/Cl2 inductively coupled
plasma (ICP) etching for current isolation purposes. The p-GaN and active
layers are partially etched by an ICP etcher to expose an n-GaN layer for
electrode formation. A indium-tin-oxide of 250 nm is deposited on p-GaN layer
as the transparent conductive layer. The samples are then annealed at 500℃
for 10 minutes in air. The Cr/Pt/Au (50 nm/50 nm/2500 nm) metals are
deposited for the p-and n-contact pads. After completing the conventional
face-up LED structure, the sapphire is ground to let sapphire thickness
become 100 μm. The Ni metal of 500 nm is deposited onto the bottom side of
sapphire substrate as the dry etching mask layer. The sample is then
subjected to the ICP process using Cl2/BCl3 (10 sccm/30 sccm) plasma with an
21
ICP power of 850 W and RF power of 400 W to form the micro pillar arrays for
light extraction purpose. The ICP etching rate for the sapphire is approximately
800 Å/min. The processed LED wafer is subjected to the laser scribed and
broken into 1000 μm × 1000 μm chips.
As for the silicon sub-mount preparation, the Ti/Al (500 Å /2000 Å) metals
are deposited onto the silicon sub-mount as a mirror. Secondly, the SiO2 film of
800 Å is deposited onto it as a passivation. The Au metal of 2 μm is deposited
for n and p bonding pad. The silicon sub-mount is subjected to stud bump
process. Figure 4-2 (a) and (b) show the top view and side view images of the
silicon sub-mount before flip-chip bonding. Finally, the chips are flip-chip
bonded on silicon sub-mount using Panasonic ultra sonic flip chip bonder for
electrical and optical measurement as shown in figure 4-3.
4-1.2 SEM images of micro pillar arrays
The surface morphology of a FC-LED with micro pillar-array sapphire
surface is examined by scanning electron microscope as shown in figure
4-4.The periodic distance for pillar-array is about 5.5 μm with the depth of the
pillar between ~1.1 μm and 3.2 μm . Furthermore, the bevel angle of pillar is
changed from 8° to 35° with increasing dry etching time. In attempt to verify the
effect of micro pillar array surface on light extraction efficiency, the various
depths and bevel angle of pillar is formed for further comparison. Figure 4-4
(a)~(b) show the rather smooth top surface and side wall and the inset is the
cross view of one micro pillar. With the increase of the dry etching time, the
surface of micro pillar becomes rougher and the bigger bevel angle is obtained
as shown in figure 4-4 (c). Even the pineapple like textured pillar surface is
obtained as shown in figure 4-4 (d). The results may be ascribed to the
22
uniformity of Ni hard mask, which results in partial over etching and the uneven
pillar surface.
4-2 Characteristics of MPAFC-LEDs
4-2.1 L-I-V measurement and efficiency enhancement
The LED chips are packaged into TO can without epoxy resin for the
subsequent measurement. The light-current-voltage (L-I-V) characteristics are
measured using a high current measure unit (KEITHLEY 240). The light output
power of the flip-chip LEDs are measured using an integrated sphere with a
calibrated power meter. The corresponding I-V characteristics of flat surface
FC-LEDs and micro-pillar-array FC-LEDs (MPAFC-LEDs) are also measured
respectively as shown in figure 4-5. It is found that the I-V curve of
MPAFC-LEDs present a normal p-n diode behavior with a forward voltage (@
350 mA) of 3.4 V, indicating that there is no heating and charging damages for
the fabrication process of micro-pillar-array during ICP etching process.
The light output power-current characteristics of the flat FC-LEDs and
MPAFC-LEDs are shown in figure 4-6. We clearly observed that the output
powers of the MPAFC-LEDs are larger than those of the flat FC-LEDs. At an
injection current of 350 mA, it is found that the MQW emission peaks of those
devices are located at about 460nm, and the light output powers of the flat
FC-LEDs, 1.1 μm, 1.8 μm, 2.7 μm and 3.2 μm depth of the MPAFC-LEDs are
about 151, 165, 179, 227 and 252mW, respectively. Figure 4-7 shows the light
extraction efficiency enhancement of MPAFC-LEDs with various depth of pillar
is 10%~68% at 350 mA current injection compared to a conventional flat
surface FC-LED. It is indicated that the textured sapphire surface reduces the
total internal reflection and improves the probability of photons escaping from
23
semiconductor to air. Furthermore, with the increase of pillar depth (1.1
μm~3.2 μm) and bevel angle (8°- 35°), the light output power of 3.2 μm
MPAFC-LED [figure 4-4 (d)] is increased by 55% compared to the 1.1 μm
MPAFC-LED [figure 4-4 (a)] under 350mA current injection. These results can
be attributed to the increase of the effective surface areas by increasing the
depth and bevel angle of micro disk.
Figure 4-8 (a) and (b) shows the images of conventional flat surface
FC-LED and MPAFC-LED under 350 mA current injection. Intensity
distributions are also shown. The EL intensities observed from the
MPAFC-LED clearly exceeded those from the conventional flat FC-LED under
the same current injection, especially on the FC-LED top surface. The
improved light extraction efficiency can be further supported by the beam view
analysis results.
Obviously, the results indicate that the sapphire substrate with
micro-pillar-array surface reduces the internal light reflection and increases the
light extraction efficiency. The probability of light escaping from the sapphire to
air is increased due to the increase of escape cone by micro-pillar-array
structure [23]. Such an enhancement can be attributed to the top surface
roughness and the fact which photons are more likely to be emitted from the
surface-roughed device, resulting in an increase of the light output power of
the MPAFC-LED.
4-2.2 Monte-Carlo ray-tracing calculations
In order to investigate the fundamental of enhancement of light output with
different etching time of MPAFC-LEDs, we used the commercial ray-tracing
software employing the Monte-Carlo algorithm to obtain trajectory of
24
ray-tracing, enhancement efficiency and spatial intensity distributions of
radiometric and photometric data.
The simulated structure and properties of FC-LEDs are shown in figure
4-9 and table 4-1, respectively. Table 4-1 (a) shows the material variable of the
models, and table 4-1 (b) shows the surface variable of the models. The
wavelength and temperature in this simulation are 460 nm and 300 K,
respectively. Figure 4-9 (a) shows the structure of the simulated models, and
figure 4-9 (b) shows the models in the TracePro software. The epitaxial layer is
not clear in the figure 4-9 (b). Figure 4-9 (c) is a sketch of the pattern on the
backside surface of the sapphire substrate. The circles in the figure 4-9 (c)
represent micro-pillar-array in these models. The distance of the center of the
micro-pillar-array is about 5.5 μm and the simulated models which are similar
to the geometric pattern of micro pillar structure as shown in figure 4-4 (a)~(d)
are designed.
The solid model is built up with combing the simple solid objects and each
semiconductor layers adjoin to the others. Light rays are generated in the
active layer with a uniform random distribution. Monochromatic radiation with
the peak wavelength of the measured spectral emission (460 nm) is used in
the simulation.
Figure 4-10 (a) and (b) shows the irradiance maps of conventional flat
FC-LEDs and 3.2 μm MPAFC-LEDs, respectively. The intensity of 3.2 μm
MPAFC-LEDs clearly exceeds that of conventional flat FC-LEDs conforming to
figure 4-8 (a) and (b).
The output power versus different depth of micro-pillar-array simulating
with TracePro software can be obtained from the irradiance maps. Therefore,
the enhancement efficiency can be calculated and it is found that the efficiency
25
is increased by larger heights of micro-pillar-array as shown in figure 4-11. The
simulated results are similar to experiment performance except 3.2 um point
because the simulated model doesn’t consider the further enhancement due to
nano roughness of micro-pillar-array sidewalls. Consequently, the improved
light extraction efficiency can be further supported by the simulation data as
shown in figure 4-11.
26
Figure 4-1 Schematic of fabrication steps of GaN LEDs with micro-pillar-arrays
27
Figure 4-2 Scanning electron micrographs (SEM) images of Si sub-mount
before flip chip bonding. (a) Top view and (b) Side view.
(a)
(b)
28
Figure 4-3 SEM image of a chip bonding on the Si sub-mount.
29
(a)
(b)
30
Figure 4-4 SEM images of micro-pillar-array surface of sapphire backside with
various depth and bevel angle. (a) 1.1 μm MPA, (b) 1.8 μm MPA, (c) 2.7 μm
MPA, and (d) 3.2μm MPA.
(c)
Bevel angleθ
(d)
31
0 50 100 150 200 250 300 3500.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Volta
ge (V
)
Current (mA)
Flat surface 1.1um MPA 1.8um MPA 2.7um MPA 3.2um MPA
Figure 4-5 The current-voltage (I-V) characteristics of flat surface FC-LEDs
and MPAFC-LEDs.
0 50 100 150 200 250 300 3500
50
100
150
200
250
Flat surface 1.1um MPA 1.8um MPA 2.7um MPA 3.2um MPA
Current (mA)
Out
put p
ower
(mW
)
0
10
20
30
40
50
60
Wall plug efficiency (%
)
Figure 4-6 The light output power-current (L-I) curves of flat surface FC-LEDs
and MPAFC-LEDs.
32
1.0 1.5 2.0 2.5 3.0 3.50
10
20
30
40
50
60
70
80
90
100
Enha
ncem
ent (
%)
MPA depth (um)
experiment (@350mA)
Figure 4-7 Light extraction enhancement of experimental results versus
different depth of MPA.
Figure 4-8 Photons of (a) conventional flat surface FC-LED and (b) micro
pillar-array FC-LED at a dc injection current of 350 mA.
(a) (b)
33
(a)
thickness index
sapphire 100 μm 1.7
n-GaN 4 μm 2.45
MQW 0.2 μm 2.65
p-GaN 0.2 μm 2.45
ITO 0.2 μm 2
(b)
reflectance transmission absorptance
Al mirror 90% 0% 10%
p-pad 50% 0% 50%
n-pad 50% 0% 50%
Table 4-1 (a) shows the material variable of the models and (b) shows the
surface variable of the models.
34
(a)
(b)
35
(c)
Figure 4-9 (a) shows the structure of the simulated models, (b) shows the
models in the TracePro software, and (c) is a sketch of the pattern on the
backside surface of the sapphire substrate.
36
(a)
(b)
Figure 4-10 The simulation results by Monte-Carlo ray-tracing. (a) and (b)
show the irradiance maps of conventional flat FC-LEDs and 3.2 μm
MPAFC-LEDs, respectively.
37
1.0 1.5 2.0 2.5 3.0 3.50
10
20
30
40
50
60
70
80
90
100
Enha
ncem
ent (
%)
MPA depth (um)
experiment (@350mA) simulation
Figure 4-11 Light extraction enhancement comparison of experimental and
simulation results versus different depth of MDA.